umm in no way shape or form would applying srs manually be > a 20 focus multi shot. I still don't understand what you meant to say in your post fully. You shouldn't use multi shot if you're gonna focus cap ? huh ? And this is only considering srs is about to fall off. So a multi shot = multi shot dmg, imp srs dmg, full duration srs refreshed, 20 focus only. And yes, you lose one free AS. But same happens with reapplying manually. It's still a GCD. Just much less dmg.

What I'm trying to say: never apply Serpent Sting to a single target via Multishot.

As the direct damage of multishot is so weak, you do more damage by reapplying SSting manually, and using that ToTH charge on an Arcane shot. It's as easy as that.
(if you don't even have a ToTH charge, you are paying 25 focus for the physical damage the multishot does. it's just stupid, as that focus invested into an arcane shot is much more damage. more than double the damage)

The scenario I mentioned is when you're at near full focus and you have ToTH procced as well as L&L and serpent sting is about to fall off.

Now since you're at full focus already, you can potentially continue to spam arcane shot until the next ES. And if I use a gcd in that time to manually cast srs, i'm still losing an arcane shot. So in both cases I lose an arcane shot. If you were at low focus then you can make that argument. That if you apply srs manually, you gain a potential arcane shot via ToTH. But at high focus it doesn't matter. Overall in the fight, you won't end up losing an arcane shot.

On the other hand if you're low on focus, obviously cobra would be the best bet. So as far as ToTH goes, it's better to use multi shot at high focus situations imo. Manually applying srs would be the choice for DB and Fervor. Although. if someone can simulate the numbers i'd be receptive to it. I could be wrong. I do get where you're coming from though.

The scenario I mentioned is when you're at near full focus and you have ToTH procced as well as L&L and serpent sting is about to fall off.

ah sorry, missunderstanding. that's what I tried to say in the other post aswell, unless you're gonna focus cap or ToTH runs out, you should not use Multi to refresh SS. but yes, in such a situation it makes sense. but that luckily doesn't happen to often :-)
thinking about it, SSting does drop off quite often these days. (even more proccs from 4p didn't help^^)
If you assume that a Cobra actually does increase SS lenth by 6sec, you can account Cobra to (indirectly) do 2 dot ticks, which is like trippling the damage a Cobra actually does... have to ponder over that, maybe changes the shot prio a bit ^^